Catalysts for purifying vehicle exhaust gas are composed of a catalytic metal such as platinum, palladium, or rhodium, and a co-catalyst for enhancing the catalytic action of the metal, both supported on a catalyst matrix made of, for example, alumina or cordierite. As such a co-catalyst are used cerium oxide-containing materials, which have oxygen absorbing and desorbing capability originated in cerium oxide, i.e., the properties of absorbing oxygen under the oxidizing atmosphere and desorbing oxygen under the reducing atmosphere. With this oxygen absorbing and desorbing capability, the cerium oxide-containing materials purify noxious components in exhaust gases such as hydrocarbons, carbon monoxide, and nitrogen oxides at excellent efficiency. Accordingly, the cerium oxide-containing materials are widely used as a co-catalyst. The property of cerium oxide is further enhanced by zirconium oxide. Thus, zirconium--cerium composite oxide is now a prevailing co-catalyst, and consumption thereof has been increasing.
It is critical for activating the function of a co-catalyst made of the composite oxide to keep the co-catalyst at a high temperature. Low temperature of the exhaust gas, for example at engine start-up, will result in low purifying efficiency. Vehicle manufacturers are presently trying to solve this problem by placing the catalyst system close to the engine for introducing hot exhaust gas right after its emission from the engine into the catalyst system.
In this case, another problem is imposed on the heat resistance of the catalyst. In general, the efficiency of exhaust gas treatment is proportional to the contact area between the active phase of the catalyst and the exhaust gas, so that the co-catalyst is required to have a sufficiently large specific surface area. However, particles of the conventional zirconium--cerium composite oxide grow when they are exposed to the high-temperature operative environment for a long period of time, resulting in reduced specific surface area. The conventional composite oxide is thus not satisfactory in heat resistance, so that co-catalysts are eagerly demanded that are capable of stably maintaining a large specific surface area.
There are proposed some methods for preparing a zirconium--cerium composite oxide having good heat resistance, for example, in JP-A-6-279027 and JP-B-8-16015. These references disclose a method including the steps of mixing a zirconium sol and a cerium sol, and adding a base to generate precipitate; and a method including spray drying. As to the heat resistance, the resulting mixed oxide is disclosed to have the specific surface area of 15 m.sup.2 /g after calcination at 1000.degree. C.
There is proposed in JP-A-5-193948 a method for preparing a mixed oxide including calcining a mixture of a hydrated zirconia sol having the average particle size of not larger than 0.2 .mu.m and a compound of Ce, Y, Ca, Mg, or the like. As to the heat resistance, the obtained mixed oxide is disclosed to have the specific surface area of 12 m.sup.2 /g after calcination at 1050.degree. C.
There is also proposed in JP-A-5-116945 a method for preparing a mixed oxide including calcining a mixture of a hydrated zirconia sol having the average particle size of 0.05 to 0.3 .mu.m and crystallite size of not larger than 4 nm and a compound of Ce, Y, Ca, Mg, or the like. As to the heat resistance, the obtained mixed oxide is disclosed to have the specific surface area of 15 m.sup.2 /g after calcination at 850.degree. C.
JP-A-5-155622 proposes a method for preparing a zirconium oxide including mixing an aqueous solution of a zirconium salt with a hydroxide, hydrated oxide, or oxide of a metal with the valency of two or more, followed by hydrolysis. As to the heat resistance, the obtained mixed oxide is disclosed to have the specific surface area of 8 m.sup.2 /g after calcination at 1000.degree. C.
All of the above-mentioned methods have the problem of long operation time for preparing the objective oxide. For example, in the method using a zirconia sol, it takes over 100 hours to hydrolyze the aqueous solution of a zirconium salt, which imposes a problem in productivity.
The large specific surface area may be achieved by suppressing the crystallite size, i.e. suppressing the growth of the crystal grains of the oxide. For example, the methods disclosed in JP-A-6-279027 and JP-B-8-16015 mentioned above using a zirconia sol (colloidal zirconia particles of 5 to 500 nm size) are optimum for this purpose. Since such fineness of the crystal grains and particles of the oxide causes extreme sensitivity to thermal energy, the oxide produced by these conventional methods is remarkably reduced in specific surface area when calcined at a high temperature in the range of 900.degree. C. or higher, and thus has poor heat resistance. Accordingly, such an oxide is not suitable for use as a co-catalyst, which is to be exposed to a high operation temperature. Further, many of the methods proposed hitherto have problems in that contamination of impurities, such as chlorine or sulfur, which originate from the starting materials and have adverse effects on the catalyst, cannot be eliminated.